Direct reading aberration-free compensator with adjustable sensitivity for use in white light interferometry

ABSTRACT

An aberration-free compensator for use in white light interferometry is provided in which anomalous fringe shifts characteristic of the difference in refractive dispersion between the two limbs of the interferometer are eliminated. The compensator is designed so that the sole difference in light paths through the compensator elements in the two limbs of the interferometer is through fluid media having refractive dispersions which are substantially identical to that of the material being studied in the interferometer. Use of such a fluid medium compensator permits changing the refractive dispersion thereof without substantially rebuilding the interferometer.

United States Patent Clark 1 Mar. 27, 1973 (54] DIRECT READINGABERRATION- OTHER PUBLICATIONS FREE COMPENSATOR WITH ModernInterferometer; Candler; Hilger & Watts Ltd; ADJUSTABLE SENSITIVITY FORUSE 195 PP- 481-482 IN WHITE LIGHT INTERFEROMETRY [76] Inventor: John B.Clark, 3203 Runkle St., Primary Examiner-Ronald wibert Niles, Mich. 49 I20 Assistant ExaminerConrad Clark Filed: y 19,1971 Attorney-E. ManningGiles et al.

[21 App]. No.: 163,862 [57] ABSTRACT Related U.S. Application Data Anaberration-free compensator for use in white light [63]Continuation-impart of Ser. No. 109,167, Jan. 25, mierferometry Providedm f anowalous frmlge I971. shifts characteristic of the difference inrefractive dispersion between the two limbs of the interferome- [52]U.S. Cl ..356/107 ter are eliminated. The compensator is designed so[51] Int. Cl. ..G01b 9/02 that the sole difference in light pathsthrough the com- [58] Field Of Search ..356/l06-1l3 pensator elements inthe two limbs of the interferometer is through fluid media havingrefractive dispersions References C'ted which are substantiallyidentical to that of the material UNITED STATES PATENTS being studied inthe interferometer. Use of such a fluid medium compensator permitschanging the refractive 3,035,482 5/1962 Kinder ..356/l07 dispersionthereof without substantially rebuilding the interferometer.

5 Claims, 5 Drawing Figures l i: Ll T:. x- E ?7 i l 34 I U 27 n EME \i 13 1-1: x 23H s. 5t K 2/ 23 PATENTEUHARZYIQYS 3,7 3, 09 SHEET 10F 3 i H Iti PRIOR ART FIGURE I.

FIGURE 2.

INVENTOR JOHN B. CLARK BY 4- FIGURE 3.

PATENIEDmzmm SHEET 2 OF 3 4v MEDUE $5 6m Pzwumuno muzmmwfia 1525 I? MOE6mm O m0 w 0 m6 wwom 03m m 221.5 4

c COMPENSATOR TRAVEL (MICROMETER DRUM INVENTOR JOHN B. CLARK awry/M7 nzATTORNjY READING, 9

PATENTEUHARZYIQYS 7 3,009

sum 3 0r 3 JOHN B. CL ARK ATTORNEY DIRECT READING ABERRATION-FREECOMPENSATOR WITH ADJUSTABLE SENSITIVITY FOR USE IN WHITE LIGHTINTERFEROMETRY BACKGROUND OF THE INVENTION This application is acontinuation-in-part of application Ser. No. l09,l67, filed Jan. 25,1971.

This invention relates to an improved compensator for used in whitelight interferometry. In one of its more particular aspects thisinvention relates to a compensator in which the problem of anomalousfringe shifts is eliminated instrumentally.

The art of interferometry provides techniques to determine differencesbetween refractive indices or thicknesses of fluids and transparentsolids by measuring the difference in the path length of light betweenthe two limbs of an interferometer, one limb containing the unknown,that is, the material upon which the determination is to be conductedand the other limb containing a reference material, the refractive indexor thickness of which is known.

In the Rayleigh interferometer, for example, shown in a schematic planview in FIG. 1, white light from a source 10 is passed through a narrowvertical slit 11, collimated by a lens 12 and passed in parallel beamsthrough two vertical slits l3 and 14 which divide the light beam intotwo beams, one of which passes through a cell 15 containing a referencematerial, the other passing through a cell 16 containing the unknown.The emergent beams from the reference and unknown cells are then passedthrough compensators 17 and 18, respectively, recombined by passingthrough collimating lens 19 and focused upon ocular cylinder lens 20forming interference fringes upon recombination. These interferencefringes can be viewed directly by means of the cylindrical ocular lensor in the focal plane of a telescope (not shown) or in the focal planeof a camera (not shown) depending upon the particular use to which themeasurement is to be put. In conducting a measurement with the Rayleighinterferometer it has been customary to use a compensator for equalizingthe light paths in the two limbs of the interferometer. One convenientmethod for accomplishing this result has been to align the interferencefringes with respect to a null fringe pattern which is produced by aportion of the light beam which does not pass through those portions ofthe cells which contain the materials being studied but instead passesthrough identical paths in both limbs. Upon placing the same material inboth the reference and unknown cell and aligning the interferencefringes thereby produced with respect to the null fringe pattern byappropriate movement of the compensator a zero setting of thecompensator is obtained as shown in FIG. 2, the lower set of fringesrepresenting the null fringe pattern. FIG. 3 shows a typical fringepattern (upper pattern) displaced from the null fringe pattern (lowerpattern). Realignment of the fringes upon introduction of an unknownmaterial into the unknown cell thereafter requires a certain movement ofthe compensator which can be measured. By appropriate calibration of thecompensator the refractive index, thickness or concentration of theunknown can be read directly.

Other interferometers such as the Jamin, the Twyman-Green, the Williams,the Michelson and the which the movable element is located. In each casea similar fixed glass plate is placed in the other limb of theinterferometer.

However, use of any of the known compensators may result in a chromaticaberration in the resulting fringe pattern if the refractive dispersionof the compensator does not match that of the material being studied.The refractive dispersion is the dependence of refractive index uponwave-length. This aberration makes alignment of the fringe pattern withthe null fringe pattern difficult because of the uncertainty inidentifying and locating the zero order fringe due to anomalous shiftingof this fringe. This observed anomalous fringe shift is characteristicof the difference in refractive dispersion between the two limbs of theinterferometer which manifests itself as a discontinuity in compensatortravel necessary to rematch the null pattern with respect to pathJengthdifference. These observed discontinuities are shown for solutions oflithium bromide and sucrose in FIG. 4 wherein compensator travel isplotted as micrometer drum reading and path-length difference is plottedas percent of solute. Each point of discontinuity causes a seriousuncertainty in the identification of the zero order fringe. In atechnique where differences on the order of a thousandth of a fringe canotherwise accurately be measured this defect has prevented the fullrealization of the potential of interferometry as a measuring means.

SUMMARY OF THE INVENTION In accordance with the present invention anaberration-free compensator for use in white light interferometry whicheliminates the observed discontinuities described above is provided.This compensator balances the path lengths of the two limbs of theinterferometer in such a way as to make the zero order fringe easily andaccurately identifiable.

In the prior art it has been suggested that a collection of compensatorplates having various refractive dispersions matched to the dispersionof the material being studied could be used for this purpose. L. H.Adams, J. Am. Chem. Soc. 37, 1181 (l9l5); L. H. Adams, J. Wash. Acad. 5,276 (1915). However, implementation of this suggestion would require theavailability of an impractically large number of different yet opticallymatched pairs of plates of high optical perfection and would entailalmost endless disassembly, reassembly, realignment and calibration ofthe interferometer involved.

The improved compensator of this invention accomplishes the desiredresult by in effect utilizing as compensator plates variable thicknessesof two fluid media, the refractive dispersions of which aresubstantially identical to that of the material being studied in theinterferometer and the refractive indices of which are different fromeach other. Such fluid media may be two solutions, the refractivedispersion of one of which is the same as that of the reference orunknown material or within the range between that of the referencematerial and unknown material, and the refractive dispersion of theother is nearly the same. The refractive dispersion of both solutionsmay fall within the range between that of the reference material andunknown material if desired. One convenient way of accomplishing thedesired result is to use the reference material itself as one of thesolutions and a material having a refractive index different from therefractive index of the reference material as the other solution.

Utilization of the improved compensator of this invention thus permitsrealization of the long recognized advantages of white lightinterferometry in differential refractometry which heretofore could notbe realized because of the difficulty and uncertainty in identifying thezero order fringe.

Various features of the invention will be illustrated and explained inthe following description of the drawing and the description of thepreferred embodiments.

BRIEF DESCRIPTION OF THE DRAWING FIG. I, described above, is a schematicplan view ofa Rayleigh interferometer;

- FIG. 2, described above, is a diagrammatic view of a typical fringepattern (upper pattern) aligned with a null fringe pattern (lowerpattern);

FIG. 3, described above, is a diagrammatic view of a typical fringepattern (upper pattern) displaced from a null fringe pattern (lowerpattern);

FIG. 4, described above, is a graph showing compensator travel plottedagainst path length difference; and

FIG. 5 is a plan view of one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 5 acompartmented compensator is shown in which the light path through onelimb of the interferometer can be varied with respect to the light paththrough the other limb by varying the ratio of the length of light paththrough one compartment of the compensator to the length of light paththrough the other compartment in one limb of the interferometer withrespect to the ratio in the other limb.

Measurementcell 21 having transparent windows 22 and 23, side walls 24and 25, barrier 26, and top and bottom, not shown, contains unknownmaterial 27 and reference material 28. Mixing of unknown material 27 andreference material 28 is preventedby means of barrier 26 which isimpermeable to the materials being studied in the interferometer. Ifdesired completely separate cells can be used for the unknown materialand the reference material.

Compensator 29 comprises front wall 30 containing transparent window 31,rear wall 32 containing transparent window 33, side walls 34 and 35,divider 36 and top and bottom, not shown. Divider 36 containstransparent windows 37 and38, left and right divider segments 39 and 40,and flexible seals 41 and 42. Positioned between the right hand portionof rear wall 32 and right divider segment 40 are springs 43 and 44.Right divider segment 40 is attached to micrometer 45 which is mountedwithin the right hand portion of rear wall 32.

Solutions 46 and 47 contained within the front and rear compartments,respectively, of compensator 29,

have refractive dispersions which are substantially identical to thoseof the reference and unknown materials. The refractive index of solution46 is different from the refractive index of solution 47. For example ifthe unknown material 27 is a sodium bromide solution having aconcentration between 1 and 2 percent by weight, then the referencematerial 28 may be a 1 percent sodium bromide solution and the solutions46 and 47 can be 2 and 1 percent solutions of sodium bromide,respectively.

Clockwise movement of the barrel of micrometer 45 causes right dividersegment 40 to move against the restraining force of springs 43 and 44 tothe extent allowed by flexible seals 41 and 42. This movement of rightdivider segment 40 causes lengthening of the light path through the rearcompartment of compensator 29 and an equal shortening of the light paththrough the front compartment in the right limb of the interferometer.No change takes place in the light path through the front and rearcompartments in the left limb of the interferometer.

Thus by movement of the micrometer the ratio of the length of light paththrough the rear compartment to the length of light path through thefront compartment in the right limb of the interferometer may be variedwith respect to the ratio of the length of the light paths through thecorresponding two compartments in left limb and the interference fringescan be aligned with the null fringe pattern as previously explained.

One advantage of using the compensator system of this invention is thatthe sensitivity of the compensator is largely dependent upon thedifference in refractive index of the two compensator solutions ratherthan the sensitivity of measurement of travel of the compensator andsuch sensitivity can be varied to meet the requirements of a particularmeasurement.

If n; is the refractive index of the reference material 28 and therefractive index of the compensator solution 47, the light path ofwhich, D is being lengthened in the right limb by an amount AD, n is therefractive index of the unknown material 27, n is the refractive indexof the compensator solution 46, the light path of which, D is beingshortened in the right limb by the same amount AD, and I is the lightpath through the measurement cell 21, the equation for the optical pathsthrough the two limbs at null alignment is:

n D n;, D n I n, (D AD) n (D AD) r1 1 Simplifying, the followingequation is obtained:

I (n m) AD (n, u

Since, in the interferometer NA: cell 2 "1),

N)\=AD (n n For one fringe:

A AD (n n Therefore:

That is, the distance the right divider segment 40 must movecorresponding to one wave length of compensated path difference dependsupon the difference between the refractive indices of the twocompensator solutions 46 and 47 and can be made sufficiently large toaccommodate the precision of any micrometer or other mechanicaltraveling device by making the difference in refractive indexsufficiently small.

Although the mathematical development has been arrived at by assumingthat the refractive index of one of the compensator solutions isidentical to the refractive index of the reference material in theinterferometer cell, a similar result will be obtained if the refractiveindex of neither of the compensator solutions is identical to that ofthe reference material so long as the refractive disperions aresubstantially identical.

Another advantage of this invention is that the compensator is directreading with respect to concentration differences Ac providing therefractive increment of the material in the cell and compensator aresubstantially identical. This can be shown mathematically as follows:

At null:

cell 2 "1) A0011 "3)- Since:

An (dn/dc); Ac,

where c concentration and (dn/dc);= refractive increment for thesolution at average concentration E,

cell 2 1) )cell AD l 3) l )compenxulorl )cell )comp.

then:

tell 2 1) AD 1 3) cell cell AD conun) Then:

cell cell) comrn) cell cmlwj cell) AD The compensator, then is directreading in Ac Although it is possible to utilize the movable compensatorelement ofv the design described above or equivalents thereof in eitherthe reference or the unknown limb ofthe interferometer it is preferredto utilize the movable element in the reference limb. In this way aparallel path change mode is effected. That is, if the refractometricpath is lengthened, the compensator path in the opposite limb should belengthened rather than shortening the compensator path in the same limbas the refractometric path lengthening. This follows from the fact thatglass and solution are less disparate 1n refractive dispersion than areair and solution and obeys the optical principle that in interferometerswhich are required to show fringes in white light, the interfering wavesshould be arranged to have as far as possible equal paths in media ofidentical dispersion.

I claim:

1. In a white light interferometer comprising a test limb containing anunknown material in the light path thereof and a reference limbcontaining a reference material in the light path thereof, acompartmented compensator which comprises means for varying the ratio ofthe length of light path through a first compartment of the compensatorto the length of light path through a second compartment of thecompensator in said test limb with respect to the ratio of the length oflight path through a first compartment of the compensator to the lengthof light path through a second compartment of the compensator in saidreference limb, each of said first and second compartments in both limbscontaining fluid media having refractive dispersions which aresubstantially identical to those of said reference material and saidunknown material, the fluid medium in said first compartment in bothlimbs having a refractive index which is different from that of thefluid medium in said second compartment in both limbs.

2. A compensator according to claim 1 wherein the refractive index ofthe fluid medium in said first compartment in both limbs is the same asthat of said reference material.

3. A compensator according to claim 1 wherein the refractive index ofthe fluid medium in said first compartment in both limbs is the same asthat of said unknown material.

4. A compensator according to claim 1 wherein the refractive index ofthe fluid medium in said first compartment in both limbs is within'therange between that of said reference material and that of said unknownmaterial.

5. A compensator according to claim 1 wherein the length of light paththrough the first and second compartments in said reference limb isvariable,

an increase in the length of light path through said first compartmentresulting in an equal decrease in the length of light path through saidsecond compartment,

the length of light path through each of the first and secondcompartments in said test limb being fixed, the total length of lightpath through the first and second compartments in said reference limbbeing equal to the total length of light path through the first andsecond compartments in said test limb.

1. In a white light interferometer comprising a test limb containing an unknown material in the light path thereof and a reference limb containing a reference material in the light path thereof, a compartmented compensator which comprises means for varying the ratio of the length of light path through a first compartment of the compensator to the length of light path through a second compartment of the compensator in said test limb with respect to the ratio of the length of light path through a first compartment of the compensator to the length of light path through a second compartment of the compensator in said reference limb, each of said first and second compartments in both limbs containing fluid media having refractive dispersions which are substantially identical to those of said reference material and said unknown material, the fluid medium in said first compartment in both limbs having a refractive index which is different from that of the fluid medium in said second compartment in both limbs.
 2. A compensator according to claim 1 wherein the refractive index of the fluid medium in said first compartment in both limbs is the same as that of said reference material.
 3. A compensator according to claim 1 wherein the refractive index of the fluid medium in said first compartment in both limbs is the same as that of said unknown material.
 4. A compensator according to claim 1 wherein the refractive index of the fluid medium in said first compartment in both limbs is within the range between that of said reference material and that of said unknown material.
 5. A compensator according to claim 1 wherein the length of light path through the first and second compartments in said reference limb is variable, an increase in the length of light path through said first compartment resulting in an equal decrease in the length of light path through said second compartment, the length of light path through each of the first and second compartments in said test limb being fixed, the total length of light path through the first and second compartments in said reference limb being equal to the total length of light path through the first and second compartments in said test limb. 